11 February 2013

Cycling performance of the different samples (the specific capacity was calculated by using the active material mass (sulfur) of the composites, given in mA h g-1). Demir-Cakan et al. Click to enlarge.

A team from the University of Picardie Jules Verne (France) and Alistore ERI (European Research Institute) has demonstrated new approaches to lithium-sulfur (Li-S) rechargeable batteries (earlier post) with improved capacity retention. An open access paper on their work was recently published in the RSC journal Energy and Environmental Science.

Li-S batteries face a number of commercialization challenges, including electrolyte-soluble polysulfides. To counter the solubility of polysulfides, other teams have pursued confinement approaches aiming to trap sulfur within the cathode side; however, success has been limited, the French team noted. Instead, they “drastically deviate[d] from this approach” and used a liquid cathode obtained by dissolving polysulfides within the electrolyte (catholytes) and also placed sulfur powders in contact with the Li negative electrode (“SLi”).

Such approaches resulted in greater performance than confinement approaches, they found. The strategy eliminates the detrimental Li2S formation inside a porous carbon matrix and moreover leads to the formation of a protective SEI layer at the Li electrode, which seems beneficial to the cell cycling performance.

The lithium–sulphur (Li–S) battery system is one of the viable options for electric vehicles to achieve a long driving range (i.e. >300 km) since the present prototypes effectively offer higher specific energy density than conventional lithium ion batteries. Despite such an appeal, Li–S batteries are not yet commercialized, the reason being that they suffer from rapid capacity fading. This poor performance comes from each cell compartment. Among them are: (i) the use of a Li metal anode which essentially brings safety problems with liquid electrolytes, (ii) the low active material utilization due to the insulating nature of sulphur, (iii) the soluble polysulphide species generated during the battery operation, which diffuse throughout the separator and deposit on the Li electrode resulting in a loss of active material, and (iv) the irreversible deposition of non-soluble lithium sulphide (Li2S) both at the cathode and Li anode.

These issues are not new as this battery technology has been extensively studied over the past five decades, nevertheless they partially remain despite intense research efforts. Recently, most of the studies have been aiming at finding the best cathode configuration via a precise control of its porosity so as to retain part of the electrochemically generated polysulphide species generated at the cathode side.

In this paper we deviate from these approaches and explore the use of chemically synthesized polysulphide species as an active material rather than sulphur impregnated composites in order to eliminate the formation of Li2S within a porous structure. Cells that have polysulphides acting as active materials are shown to present superior performance to the conventional Li–S cell configuration. Moreover, we found that these performances can even be further enhanced when sulphur is directly deposited on the Li negative electrode.

—Demir-Cakan et al.

For the study, the team used 1 M lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) containing tetra-methylene sulfone (TMS) as the electrolyte. Polysulphides with a nominal formula of Li2Sx (x>2) were chemically synthesized and dissolved in the electrolyte (catholyte) and used directly as an active material. Sulfur loading in the polysulfide was carefully tuned.

In the study, the researchers compared the performance of two different catholytes made by dissolving polysulfides of different lengths (Li2S2 and Li2S8); the SLi approach; and a mesoporous C–S composite.

Targeting the origin of the rapid capacity decay in Li–S batteries is a must if we ever want this system to become a reality for load leveling and transport. To address this issue we report on two different approaches enlisting the use of either polysulphides as active materials or S deposited on Li, both aiming to eliminate the detrimental formation of Li2S at the porous carbon matrix. Besides leading to performance improvements in terms of capacity retention, these approaches have also led to better insights regarding the impact of sulfur deposited on the Li surface. Via the use of EIS spectroscopy, we have shown evidence for the growth of a specific SEI which can act as a self-limiting barrier for chemical reactions while enabling to carry Li-ions upon application of an electrical field.

To conclude, this SEI layer seems to combine attractive features, hence the crucial importance to understand both its nature and composition and to pursue more intensive chemical/physical analysis enlisting combined XPS-NMR surface analytical techniques. Although not fully understood, such a finding, which somewhat mimics what happens in Li–thionylchloride primary cells, holds some promises regarding the feasibility to build Li–S cells differently for sustainable performance.

Comments

The positive news about this experiment is the information derived from the attempt. It says this approach doesn't work thus saving other chemists from wasting time trying it.

Frankly, I think our best attempt currently to develop a more dense battery is with the DOE JCESR project. I worry how easily breakthrough patents from labs can be purchased and sequestered by Big Industries to slow down EV progress, to extend their profits and to continue their control over the U.S. energy market. This won't be the case when the developments are so much in the Public eye and run by a Government Lab. In this case Argonne Labs.

@ToppaTom, "breakthrough patents from labs are purchased and sequestered by Big Industries to slow down EV progress” came from GM selling the NiMH patents to be BURIED by Chevron Oil while Toyota turned grandfathered <10 amp/cell NiMH use into over 4 million Prius hybrids - and counting.

At just 4 million X $25,000/Prius, that's $25 billion($25,000,000,000) for Japan and the US taxpayer got to give GM a $50 billion bailout.

But Stan Ovshinsky himself (inventor of the leading form of the rechargeable NiMH batteries), when asked, "So it’s your opinion that Cobasys is preventing other people from making it [large-format NiMH batteries] for that reason?", he responded "Cobasys is not preventing anybody. Cobasys just needs an infusion of cash."

Cobasys was a joint venture of ECD Ovonics and the ChevronTexaco Corporation.

At 1 million Prius sold in the US (up to the end of 2011) x $3,000/Prius (rebates that the US taxpayer paid), that's $3 billion of our tax dollars for Japan.

Without rebates the sales would have been MUCH, MUCH more dismal than they were (dismal for Toyota, not necessarily for the taxpayer).

kelly....the real problem may be closer to home. It seems that we are no longer able to compete will others, specially making high quality batteries (and large aircraft! etc)

The days when most of the world was looking to USA for high quality affordable products came to a progressive end a few years ago? USA is now looking at Asia and EU to buy high quality affordable products (often with borrowed money at the rate of about $500B/year?).

By "Cobasys is not preventing anybody. Cobasys just needs an infusion of cash." your saying the Chevron oil partner was broke? Chevron licensed Panasonic to CONTINUE making EV95(RAV4 EV) batteries?

Ovshinsky said, "I think we at ECD made a mistake of having a joint venture with an oil company, frankly speaking. And I think it's not a good idea to go into business with somebody whose strategies would put you out of business, rather than building the business."[15] http://en.wikipedia.org/wiki/Patent_encumbrance_of_large_automotive_NiMH_batteries

"ChevronTexaco also maintained veto power over any sale or licensing of NiMH technology."

"the United States with 12,750 units sold through December 2012," plug-in Prii(the rebate ones, which qualifies for a $2,500 Federal Tax Credit) were sold. http://en.wikipedia.org/wiki/Toyota_Prius_Plug-in_Hybrid

TT says, "that's $3 billion of our tax dollars for Japan".

Fact, 12,750 Prii Plug-ins X $2,500 = $31,875,000 IF ALL FULL tax credits were given to US taxpayers.

$3,000 million vs $31.875 million: 3,000/31.875 = >94

In other words, ToppaTom lies by a factor of at least 94 to 1.

TT, you truly are the, "There are too many crackpot conspiracy theories to keep track of."

What a blog post!! Very informative and also easy to understand. Looking for more such comments!! Do you have a facebook? I recommended it on digg. The only thing that it’s missing is a bit of new design. Here

Last night the State of the Union speech could have added that four years of US energy policy is clearly bearing fruit.

During eight years of Bush, 4,000 1990s EVs on the road were CRUSHED, as were the CARB laws encouraging them.

Now, over 4,000 EV's are being BUILT A MONTH.

Five years ago, Tesla was a ailing consulting company - not having sold a car to the public(not counting Elon's) till September 2008.

Now, the Tesla Model S is the Motor Trend Car of the Year. At a profitable 400 cars a week, more Tesla Model S are being sold than Mercedes Benz S Class - and they had a over a hundred year head start.

Huge advances in solar cells, wind turbine, genetics, bio-fuels, batteries, including the article Li-S, etc, and on and on are just beginning to reach the public.

TT, (like many posters) do not (yet) see the real advantage of future electrified lighter vehicles, but given enough time, they will figure it out sooner or latter.

The transition rate will soon reach 8% to 10%/year in many countries. Japan has already done much better. China will follow soon to solve part of their air pollution problem. EU will wake up by 2015 or so. USA is progressing slowly but will catch up sooner or latter. Canada will not move an inch until a new government is elected in October 2015.